1. Field of the Invention
The present invention relates to a driver of a display unit such as a liquid crystal display, an organic light emitting display, a plasma display or the like. More particularly, the present invention relates to a driver such as a column driver, a source driver, or a horizontal driver or the like.
2. Description of Related Art
A display unit has recently become larger and larger in size due to development of manufacturing technique. The display unit having large size requires ability of driving large capacitance load of output of the driver. It means that output impedance of the driver needs to be decreased. If the output impedance is not substantially small, there is caused a problem such as lack of driving ability, increase of power consumption, or heat generation.
Further, recent display unit performs multi-gradation display, and there is developed a multi-bit driver of the display unit. Moreover, the driver of typical display unit has hundreds of driving outputs and includes latch circuits, level shifters, D/A converters, and buffer amplifiers.
FIG. 13 shows an example of a driving output circuit in the driver of the display unit according to a related art.
The driver shown in FIG. 13 is an output circuit having two outputs. A driver 10 of the display unit includes a latch circuit 11, a level shifter 12, a D/A converter 13, an output amplifier 15, an output switch 16, and an output pin 17. In this example, it is assumed that the display unit is a liquid crystal display and includes a polarity switching circuit 14 and the output switch 16. In this example, the polarity switching circuit 14 is provided between the D/A converter 13 and the output amplifier 15. However, the polarity switching circuit may be provided between the output amplifier 15 and the output pin 17. In this case, the polarity switching circuit may also function as the output switch.
Hereinafter, a behavior of the driver of the display unit shown in FIG. 13 will be described in brief. The latch circuit 11 holds digital gradation information for each driving output and outputs the digital gradation information to the level shifter 12 as the output signal. The level shifter 12 performs voltage level conversion between the latch circuit 11 which is a low voltage circuit and the D/A converter 13 which is a high voltage circuit. The digital gradation information output from the level shifter 12 is converted into gradation information signal having analog value by the D/A converter 13 according to its digital value. The gradation information signal having analog value that is output from the D/A converter 13 is alternately switched by the polarity switching circuit 14 in a predetermined cycle and input to the output amplifier 15. The output amplifier 15 amplifies the analog gradation information signal and outputs the amplified signal to the output pin 17 when the output switch 16 is in ON state.
In the multi-bit driver as stated above, the test may take longer time and accuracy is not high. In order to overcome these problems, Japanese Unexamined Patent Application Publication No. 2006-227168 discloses a technique to provide a driver of a display unit in which inspection time is reduced and inspection accuracy is improved.
In the prior art disclosed in Japanese Unexamined Patent Application Publication No. 2006-227168, the driver includes a selector selecting output of the latch circuit to output latch data from a predetermined bit, and an output selector switching a level shifter output corresponding to the predetermined bit and gradation voltage output. In normal operations, the selector is switched so as to output the gradation voltage to the driving output pin. In test operations, the selector is switched so as to output voltage (test output voltage) according to the level shifter output corresponding to the predetermined bit.
As stated above, the display unit having large size requires ability of driving large capacitance load of output of the driver. If the output impedance of the driver is not substantially small, there is caused a problem such as lack of driving ability, increase of power consumption, or heat generation.
In the prior art as in Japanese Unexamined Patent Application Publication No. 2006-227168, there is provided an output selector where gradation voltage is output to the driving output pin of the driver in the normal operation and test output voltage is output in the test operation. This output selector needs to be composed of the transistor since the output selector is implemented in the integrated circuit. The switch made of transistor has impedance in accordance with its size. Therefore, if the transistor having lower impedance is employed in order to maintain large driving ability, the size of the integrated circuit composing the selector increases. On the other hand, if the selector is composed of small transistor in order to avoid increase in size, the output impedance increases and the ability of driving the output load is lacked. Further, if the driving ability of the amplifier is enhanced in order to compensate lack of driving ability, there are caused other problems such as increase in power consumption and heat generation.
Therefore, there is a need to connect the driving output pin and the test signal without directly adding the selector which is one of factors for increasing impedance to the driving output pin which requires driving ability.
There is Japanese Unexamined Patent Application Publication No. 2006-053480 as a prior art.